1. Field of the Invention
This invention relates generally to voltage regulators internal to integrated circuits and, more particularly, to an nV.sub.BE type voltage regulator having an NPN shunt transistor for providing improved regulation.
2. Background Art
The prior art is replete with various voltage regulator circuits for supplying substantially constant output DC regulated voltages. There are as many techniques for regulating the voltage output of these regulator circuits as there are applications for such regulators.
Known techniques for voltage regulation include bandgap regulators and simple diode strings, or variously refined versions of diode strings all known as nV.sub.BE regulators.
Bandgap voltage regulators vary widely in specific design but the basis of a bandgap regulator comprises a first diode having a cathode coupled to a negative power supply voltage terminal and an anode coupled to a first node. A plurality of NPN transistors have their bases coupled to the first node and their emitters coupled to the negative supply voltage terminal through a first resistor. The collectors of the plurality of NPN transistors are coupled to an output node through a second resistor. The first node is coupled to the output node through a third resistor. A second NPN transistor has its collector connected to the output node, its base connected to the collectors of the plurality of NPN transistors, and its emitter coupled to the negative supply voltage terminal. The output node is coupled to the positive supply voltage terminal through a fourth resistor.
The disadvantage of a bandgap regulator for internal voltage supply use is that the temperature coefficient of the output voltage is dependent upon the value of the output voltage. Additional circuitry must be added to achieve both the desired output voltage and the desired temperature coefficient, thereby increasing circuit complexity and chip area. Additionally, the output voltage of a bandgap is very sensitive to extraneous emitter resistance in the plurality of NPN transistors. Process variations can routinely cause the output voltage to be out of specification due to the mechanism of emitter resistance.
Diode string regulators generally comprise a plurality of diodes coupled in series between a first supply voltage terminal and a node. A resistor is coupled between the node and a second supply voltage terminal. The polarity of the supply voltage is such that the diodes are forward biased. The base of an output emitter follower transistor is connected to the node and the emitter provides the output. The output voltage of a diode string regulator or of any nV.sub.BE regulator varies with temperature as some multiple (n) times the variation of a diode voltage drop (V.sub.BE) with temperature. This desirable characteristic known as nV.sub.BE temperature tracking permits cancellation of temperature effects within the circuitry that is driven by the nV.sub.BE regulator. The simplicity and nV.sub.BE tracking make the nV.sub.BE regulator an excellent choice for internal voltage regulators.
The disadvantage of a diode string regulator is inadequate voltage regulation with respect to the power supply voltage. Changes in supply voltage can cause large changes in current through the diode string, changing the diode voltage drop and thus the output voltage. Thirty to forty millivolts per volt is a typical regulation and is sufficient to cause excessive variation in chip power with supply voltage variation.
One previously known nV.sub.BE regulator comprises an NPN transistor having an emitter connected to a negative supply voltage terminal, and a collector coupled to an output node by a first resistor. The output node is coupled to a second supply voltage terminal by a second resistor. The base of the NPN transistor is coupled to the negative supply voltage terminal by a third resistor and is coupled to the output node by a fourth resistor. The voltage from the base to the emitter of the NPN transistor is the reference voltage, V.sub.BE, of the regulator. The resistance of the fourth resistor is some number (n) times the resistance of the third resistor and so sets the value of the output voltage at (n+1)V.sub.BE. The output node generally provides an output by driving an emitter follower transistor. A PNP transistor has an emitter connected to the output node, a collector connected to the negative supply voltage terminal, and a base connected to the collector of the NPN transistor. The PNP transistor is intended to clamp the current through the NPN transistor at a constant value over variations in supply voltage. Constant current through the NPN transistor implies a constant value of V.sub.BE and thus a constant regulator output voltage.
However, the type of PNP transistor compatible with modern digital bipolar processing requires a large area on the chip and has a low beta. As a consequence of the low beta, the PNP transistor has a very high base current which flows into the collector of the NPN transistor. Thus a large portion of current variation that is intended to be shunted around the NPN transistor actually still flows through the NPN transistor, causing variation in V.sub.BE and regulator output voltage. Twenty to thirty millivolts per volt is typical regulation, inadequate for modern circuitry.
Thus, a need exists for an improved integrated voltage regulator circuit having an NPN shunt transistor for improving regulation and reducing chip size.